The present invention relates to the fabrication of printed circuits, and more particularly to solder masks utilized in connection with printed circuit fabrication.
A solder mask comprises a resin coating which is provided on a printed circuit board surface in order to prevent all but selected areas of the board (e.g., metallized through-holes and their surrounding pads; lands for surface mounted electronic components) from coming into contact with solder during operations which are designed to selectively provide solder at those areas (e.g., by tin-lead reflow/fusing or hot air solder levelling) and during operations which are designed to solder-connect electronic components at those areas (e.g., wave soldering). Solder masks also serve as a means for generally protecting surface circuitry against scratches, dirt contamination, corrosion, and the like.
Of necessity, solder masks must be applied to the board surfaces in a predetermined pattern so that holes, pads, lands, etc. are not covered thereby. One means for effecting this patterned application is to provide a liquid resinous composition which can be cured (cross-linked) either by heat or by radiation-induced mechanism. The composition is screen printed onto the board directly in the desired pattern through an appropriately-stenciled screen, and then baked or blanket-exposed to radiation (e.g., UV), as the case may be, in order to cure the patterned composition into a suitable solder mask. An advantage of such solder mask compositions is that they can be formulated as essentially 100% solids compositions, i.e., liquid compositions but without need for a volatile solvent carrier which might pose health concerns in handling of the compositions and during evaporation after application.
A disadvantage of screen-printed solder mask compositions, however, is the difficulty in applying them in the precisely defined patterns dictated by modern day fine line, closely-spaced circuitry. The compositions tend to bleed through the stencil onto board areas where solder mask is not desired, necessitating constant monitoring and touch-up in order to blot away such areas before final cure takes place.
Much improved fine line definition is attainable utilizing photo-sensitive solder mask compositions which are applied in generally unpatterned form (e.g., as a continuous coating) over the circuit board surface, followed by exposure of selected areas of the composition to radiation of suitable wavelength so as to effect radiation-induced curing of the composition at such selected areas. A developer is then employed to remove unexposed composition and produce the desired solder mask pattern.
In known methods for utilizing photo-sensitive solder mask compositions, the imagewise exposure to radiation is accomplished using an appropriately patterned phototool which permits transmission of radiation (e.g., UV light) therethrough, to the composition coating, only in the desired pattern, and for the most precise line definition, the phototool must be arranged in direct contact with the composition. For this reason, 100% solids-type compositions cannot be employed, i.e, it is not possible to place a phototool in direct contact with these wet (liquid) compositions. Rather, the photo-sensitive solder mask composition must be formulated as a solvent-carried resin system which is applied to the board as a liquid coating and then evaporated to substantial dryness so as to enable arrangement of the phototool directly thereon. In turn, then, this necessity brings into play the earlier noted problems and concerns with volatile solvents.
Dry film solder masks avoid, at least for the printed circuit manufacturer, the need to deal with solvent-carried photo-sensitive solder mask compositions. However, dry films are inherently more expensive than liquid photo-sensitive compositions, and require more expensive application techniques and equipment.